1. Field of the Invention
This invention relates to a semiconductor laser, and an optical pickup device and an optical record and/or reproducing apparatus using the semiconductor laser.
2. Description of the Related Art
FIGS. 1A and 1B show a conventional real index-guide semiconductor laser. FIG. 1A is a schematic view showing of the conventional real index-guide semiconductor laser, and FIG. 1B is a schematic view showing distribution of refractive index corresponding to FIG. 1A.
As shown in FIG. 1A, the conventional real index-guide semiconductor laser includes an n-type cladding layer 101, active layer 102 and p-type cladding layer 103 on a semiconductor substrate (not shown). An upper-layer portion of the p-type cladding layer 103 has the form of a stripe extending in one direction and having a given width. The extending direction of the stripe portion 103a corresponds to the cavity-lengthwise direction of the real index-guide semiconductor laser. Formed in opposite sides of the stripe portion 103a are low-index regions 104 having a lower refractive index than that of the p-type cladding layer 103.
FIG. 1B shows distribution of equivalent refractive indices of the conventional real index-guide semiconductor laser. As shown in FIG. 1B, the conventional real index-guide semiconductor laser has a stepped distribution of refractive indices where the refractive index is higher at a portion aligned with the stripe portion 103a and lower at portions aligned with the low index regions 104 in opposite sides of the stripe portion 103a. That is, it has one equivalent index step in the direction parallel to the p-n junction and perpendicular to the cavity-lengthwise direction. In this manner, stabilization of the transverse basic mode of laser oscillation is intended.
In the conventional real index-guide semiconductor laser, the stripe portion 103a of the upper-layer portion of the p-type cladding layer 103 is made by first growing the p-type cladding layer 103 on the entire surface and thereafter etching the p-type cladding layer 103 by wet etching. However, due to an unsatisfactory controllability of the etching rate during the etching process, the stripe portion 103 results in variance in width (stripe width). More specifically, the variance amounts to .+-.0.3 .mu.m or more. This results in a variance as large as .+-.2.degree. or more in horizontal radiation angle .theta.// of laser light depending on the stripe width.
Another problem occurs when the conventional real index-guide semiconductor laser is used as a light source of an optical pickup device or an optical record and/or reproducing device. That is, when the horizontal radiation angle .theta.// of laser light exceeds the lower limit, the amount of peripheral light of the lens decreases, and the light spot is expanded on the surface of an optical disc. When, in contrast, the horizontal radiation angle .theta.// of laser light exceeds the upper limit, the semiconductor laser needs a larger power for emission of light during recording on the optical disc, and may cause undesirable evaluation to the reliability of the semiconductor laser.